BAGHDAD, Dec. 8 -- A silver Mercedes swings into the passing lane when a machine gun opens fire, sending the car smashing into a taxi, whose terrified occupants scatter. Moments later on the video, posted on the Internet and apparently recorded in Iraq, a white sedan is riddled with bullets as it accelerates on an open highway.

Framed as if on a movie screen by the outline of a sport-utility vehicle's rear window, those scenes and others show what appear to be private security contractors firing on Iraqi civilians. The video footage has prompted an investigation by the U.S. military, a spokesman said Thursday, and by the company linked to the incidents. It even has a soundtrack: Elvis Presley's upbeat "Mystery Train."

Details about the origin of the video clip and the location shown in it are unknown. It was originally posted last month on a Web site maintained by former employees of Aegis Specialist Risk Management, a London-based company that has a $293 million U.S. government contract to provide security services in Iraq. The video has since been removed from the site.

"Aegis has established a formal board of enquiry, in cooperation with the U.S. military authorities, to investigate whether the footage has any connection with the company and, should this prove to be the case, under what circumstances any incident took place," the company said in a statement about the incident.

A public relations representative for Aegis said the company's findings could come within the next week.

"An investigation has been initiated, but we do not have any details at this time," Army Capt. Bill Roberts, a U.S. military spokesman, said in an e-mail message Thursday.

There are more than 25,000 private security contractors working in Iraq, according to industry estimates. In an effort to limit the number of U.S. soldiers in Iraq, the military employs private contractors to handle jobs that would otherwise be performed by troops. But the conduct of security contractors has occasionally come under scrutiny, and Iraqi civilians and military commanders have charged that they shoot indiscriminately and flout local laws with impunity.

The companies, whose employees have been frequent targets of insurgent attacks and perform some of the country's most dangerous jobs, such as guarding highway convoys, maintain that they use force only when necessary for protection. The rules of engagement "allow for a structured escalation of force to include opening fire on civilian vehicles under certain circumstances," Aegis said in its statement about the video.

Aegis typically performs 100 "escort assignments" per week on roads in Iraq, according to its Web site.

But many Iraqis complain that the force used by contractors, who are immune from prosecution under an order signed into Iraqi law last year, is often excessive.

"At least the police and army are recognized in the street, and they have the right to shoot because they are security forces," said Qasim Muhammed, 44, a Baghdad taxi driver. "But who gave those civilians the right to shoot?"

The newly released video, which was broadcast widely on Arabic-language satellite television stations in recent days, shows no faces and contains few audible bits of dialogue. Because of that, identifying those involved will be difficult, a U.S. military official said on condition of anonymity.

The video contains four segments that appear to have been shot from the type of vehicle often used by security companies, and which often have cameras mounted in the back. In some of the segments, it cannot be determined where the bullets are striking. It is also not possible to determine whether anyone was injured in the shootings.

In the first segment, a man's voice can be heard, along with the music, saying, "These two aren't stopping," as a white sedan emerges from the traffic and travels toward the camera. Machine-gun fire erupts, and the white car slows down. Then a van swings into the passing lane and begins to accelerate, and the machine gun sounds again, continuing to fire as the van fades into the distance.

The scene quickly shifts to the silver Mercedes on a rural highway. In the third part of the video, someone in the vehicle with the camera throws what appears to be a smoke canister toward oncoming traffic, in an apparent attempt to encourage those vehicles to slow down. When they continue to advance, the vehicle with the camera slows to a crawl and the machine gun fires. An approaching red sedan swerves off the right side of the road.

The final segment shows the white sedan being riddled with bullets, as a convoy of U.S. military Humvees passes about 60 feet away on a parallel highway.

Founded three years ago, Aegis is run by Tim Spicer, a former lieutenant colonel in the Scots Guards, a British army unit. A previous firm run by Spicer, Sandline International, was disbanded in the late 1990s after it was accused of breaking an embargo on the sale of arms to Sierra Leone.

The site where the video was first posted ( http://www.aegisiraq.co.uk/ ) does not belong to Aegis, according to a statement on its main page. "It belongs to the men and women on the ground who are the heart and soul of the company," the statement says.

Another message on the site is said to have been posted by Spicer. "Remember that your job and those of your colleagues indirectly relies on the maintenance of our contract," it reads. "Refrain from posting anything which is detrimental to the company since this could result in the loss or curtailment of our contract with resultant loss for everybody."

Although the video has been removed, its contents were debated on the message board attached to the Web site.

"although i haven't viewed this footage ive read a lot of posts condenming it. All i can say is if you havent been there you dont have a say," an anonymous poster wrote. "Its the lonliest job on the planet as im sure guys will agree. You and you alone have to make the decision wether to open fire or not."

Other videos apparently shot by contractors have been made public in recent months. One circulating in Baghdad shows contractors returning fire after being ambushed on the road to Baghdad's international airport. At least one contractor in the video appears to have been killed.

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